Optimal Length of Hybrid Metal-Semiconductor Nanorods for Photocatalytic Hydrogen Generation

Hybrid metal-semiconductor nanostructures have been utilized as attractive model catalysts for understanding photocatalytic reactions because each geometrical factor is precisely controllable. Herein, we prepared Pt-tipped CdSe nanorods and tailored their length. The maximum hydrogen-evolution rates were obtained in the length of 15-20 nm for the single-tipped nanorods and 30 nm for the double-tipped nanorods. By means of time-resolved spectroscopic analysis and kinetic modeling, we revealed that the hydrogen-evolution rate was directly proportional to the amount of long-lived charge-transfer state dominated by the interplay between the carrier diffusion to the metal center and recombination. As the length increased, the absorption cross section increased, whereas the dissociation rate of excitons decreased. As a result, the number of carriers migrating to the metal tips was maximized with the 15-20 nm nanorods per tip. This information provides a direct guideline to design the optimal geometrical configuration of metal-semiconductor hybrid catalysts.